The use of NMR techniques in measurement, detection and imaging has become desirable in many scientific fields of endeavor. The non-invasive, non-destructive nature of NMR has facilitated application to industrial instrumentation, analysis and control tasks, in a variety of applications, including but not limited to cosmetics, perfumes, industrial chemicals, biological samples and food products. As one example, check weighing is used by the pharmaceuticals industry for monitoring and regulating the amount of drug in a sealed glass vial during filling. The drug weight can be as small as a fraction of a gram, and is required to be weighed with an accuracy of a few percent or better, in a vial weighing tens of grams at a rate of several weighings per second.
International Patent Application No. WO 99/67606, incorporated herein by reference as if fully written out below, describes a check weighing system for samples on a production line using NMR techniques. This system includes a magnet for creating a static magnetic field over an interrogation zone to produce a net magnetisation within a sample located within the interrogation zone, and a RF coil for applying an alternating magnetic field over the interrogation zone to cause excitation of the sample according to the principles of NMR.
As is well known in the NMR art, after pulse excitation of the sample by the alternating magnetic field, the sample emits a signal induced in the RF coil, called the free induction decay (“FID”), from which much information, like sample mass (or weight) can be learned. The FID is directly proportional to the net magnetisation applied to the sample. Consequently, any variation in the applied magnetisation produces changes in the FID, including its frequency and spatial orientation, and effects the determination of sample weight obtained from the FID. When a single NMR measurement is being made, a NMR spectrometer may be manually calibrated and proper results achieved. However, when multiple NMR measurements are being made over time, as is the case when weight checking of containers in a production line is in continuous operation, the magnetic fields drift because of temperature variations in the magnets used to generate those magnetic fields. Thus, in such applications it is imperative to monitor and correct for variations in the magnetic fields. Adjusting the NMR magnetic field in this manner insures that the resonance frequency remains at the resonant frequency of the sample, and improves the accuracy and precision of the determined weight.
It is desirable to provide a method for insuring that the characteristics of the magnetic field used in a NMR check weighing system for samples on a production line track deviations from the resonant frequency of the sample.